In the wastewater treatment industry, it is known to remove impurities from sewage using a microbial oxidative digestion process. During this process, the action of certain bacteria and microorganisms can decompose or digest biodegradable compounds (commonly referred to as BOD) located in sewage into non-harmful compounds. These organisms are aerobic in nature and require oxygen to function and survive; thus, a sufficient amount of dissolved oxygen (D.O.) must be present in the sewage to sustain their life processes.
Sewage, in particular, has increased organic water pollution which correspondingly supports a high microbial population. For example, domestic wastewater commonly has BOD concentrations in the range of 100 to 300 mg/L as compared to the acceptable BOD concentration of a natural body of water of less than 9.0 mg/L. Due to this increased BOD concentration, a much higher microbial oxygen demand is created in sewage that cannot be satisfied by natural oxygen aeration processes. If the D.O. demand is not met, hydrogen sulfide (an odorous gas) creation readily occurs and microbial organic breakdown is limited. Therefore, it is known by those skilled in the art to artificially oxygenate the sewage to promote the activity of the aerobic microorganisms located therein.
The desire to increase D.O. levels is not limited to wastewater treatment applications. It is also applicable to slow moving bodies of water (such as the waterways of Chicago, Ill.) and other fluids. Insufficient D.O. levels in rivers, streams, and canals in which treated wastewaters are discharged can compel entire industries to curtail production to considerable economic detriment. Low D.O. levels also can result in harm to the local ecosystem as fish and submarine flora require D.O. to function. Moreover, systems for dissolving a gas into a fluid are not limited to dissolving oxygen in water. Other gas/fluid combinations include: hydrogenation of vegetable oils, coal liquification, yeast production, Vitamin C production, pharmaceutical and industrial aerobic bioprocesses, chemical oxidation of sulfide, and other processes well known in the art.
The activated sludge process employs the above concept and is the most commonly used biological treatment method. Conventionally, the wastewater is pumped into an aeration tank where it is infused with oxygen from the air, thereby adding dissolved oxygen (D.O.) into the wastewater. Introduction of air into the sewage intimately mixes the organic components of the sewage with the bacteria populations in the aerator chamber, creating an activated sludge. By aerating the wastewater, the environmental conditions in the aeration tank are maintained so as to promote the optimal growth of microorganisms and, thus, achieve maximum BOD removal. In conventional activated sludge systems, D.O. is supplied by either bubbling air through diffusers in the bottom of the aeration tank (submerged diffusers) or splashing the water in contact with the air (surface aerators). An enormous amount of air has to be supplied to such systems to satisfy the bacterial oxygen requirements.
It is well known that pressure and an increased oxygen composition of the gas greatly enhances the dissolving of oxygen into a liquid, including, but not limited to, dissolving air or high purity oxygen (HPO) into water. Therefore, it is commonly known to those skilled in the art to pressurize the oxygen transfer reactor in order to increase D.O. dissolution. For instance, with 100% oxygen gas and a pressurization of 12 atmospheres in the gas transfer reactor, it is possible to increase the D.O. concentration of sewage to over 500 mg/L. Moreover, using a pressurized oxygen transfer reactor does not significantly increase the energy requirements of the system. It can be shown that the unit energy consumption (kwhr/ton of gas dissolved) in a pressurized gas transfer system is constant as the pressure within the oxygen transfer reactor and amount of dissolved gas in the fluid discharge increases. However, it is difficult to maintain the high D.O. concentration within the fluid because once the supersaturated fluid is no longer under the increased pressure of the gas transfer reactor, effervescence readily occurs. Therefore, due to the loss of D.O. through effervescence, the high energy expenditure that is required to dissolve the large amounts of oxygen into the fluid is all but wasted.
In the reactor/aeration tank of prior art systems, microorganisms are suspended and the contents are subjected to a turbulent regime to maintain the suspension. If the bacteria settle, the organic components are not as accessible and are less likely to be metabolized by the bacteria. Generally, wastewater containing suspended and dissolved organics is introduced at the inlet end where it is mixed with the returned sludge and discharged into the tank. The tank contents, including wastewater, returned sludge, and suspended biological floc, are known as mixed liquor. The mixed liquor is continuously withdrawn from the tank and residence time in the tank is varied to achieve the desired treatment efficiency. In conventional systems, the required D.O. is typically added over a 2 to 8 hour residence time in the aeration tank.
Once the sewage has been treated sufficiently, i.e., when the bacteria have broken down a target amount of organic components in the solution, the mixed liquor is discharged into a secondary clarification chamber where the biological solids settle down to form sludge, and the clarified water overflows over the effluent barrier. A portion of the sludge is recirculated to the aeration tank to maintain a steady concentration of BOD removing microorganisms in the tank and any excess sludge is wasted. One disadvantage of oxygenating a recirculating side-stream of recycled mixed liquor in a pressurized gas transfer reactor is that the microbial consortium contained therein is subject to intense shear by the hydraulic turbulence in the depressurization zone (the microbial reactor/aeration tank being pressurized versus the non-pressurized oxygen transfer chamber). This can adversely affect microbial flocculation or solids retention within the system used to maintain the bacteria or biomass within the reactor.
The key to consistent, efficient, and reliable sewage treatment is assuring that the proper amount of D.O. is introduced into the sewage for reaction with the microorganisms or bacteria. Even employing this aeration technique, it is common for oxygen transfer to limit the overall process. This is especially true considering the prospect of utilizing immobilized biomass carriers or membrane retention for the purpose of greatly increasing the amount of microorganisms, or biomass, present in the biological reactor. Given that domestic wastewater commonly has BOD concentrations in the range of 100 to 300 mg/L, this BOD concentration level is nominally the concentration of D.O. that must be supplied, depending on the solid retention time used in the process. Under conventional conditions utilizing aeration, the requisite D.O. demand is added to the wastewater over 4 to 8 hours of residence time in the aeration tank. This residence time coincides with the time it takes for the bacteria to metabolize the BOD. Thus, if 300 mg/L of D.O. is to be dissolved over a 6 hour period of time, this amounts to slightly less than 1 mg/L-min.—a relatively slow rate of D.O. addition to the system.
Conventionally, the concentration of microorganisms in the aeration tank is 1,000 to 3,000 mg/L and their D.O. uptake rate is relatively low; comparable to 1 to 2 mg/L-min. Using conventional aeration, the saturation concentration of D.O. in contact with air is about 9 mg/L. However, it is desired to maintain the D.O. in contact with the bacteria at 1 to 2 mg/L, which is adequate to sustain unrestricted bacterial metabolic rates. The rate of gas transfer, dc/dt, is related to a number of factors as shown in the gas transfer equation:dc/dt=K1(A/V)(Csat−Cact)where                K1 is the oxygen transfer coefficient        A is the interfacial area of gas exposed to the water        V is the volume of the water        Csat is the saturation concentration of D.O. in contact with gas        Cact is the actual concentration of D.O. in the water        (Csat−Cact) is the D.O. deficit driving force concentrationUnder conventional activated sludge operation, the D.O. deficit is 9−2 mg/L=7 mg/L. If the D.O. concentration maintained in the wastewater is higher than 2 mg/L, the D.O. deficit driving force concentration decreases and thus the rate of oxygen transfer decreases, resulting in increased unit energy consumption per unit of D.O. dissolved. In other words, these systems are most efficient when the D.O. concentration in the water is low, and are progressively more inefficient as the water D.O. level approaches saturation. For this reason, in conventional aeration systems the D.O. in the wastewater is maintained at the lowest D.O. concentration that does not restrict the rate of metabolic activity by the bacteria. Thus, conventional aeration systems allow for only limited D.O. increments in an effort to keep the system from being cost prohibitive.        
Conventional aeration systems also do not provide a means to maintain adequate D.O. concentrations required for optimal BOD removal in the newer technologies that allow for a higher concentration of bacteria (MLSS) to be maintained in the reactor. At low food-to-microorganism (F/M) loading rates of 0.1 to 0.2 lb BOD/lb MLSS-day, only about 1 mg/L of D.O. is necessary to allow unrestricted bacterial metabolic activity. However, as the loading rate to the reactor increases, the D.O. in the bulk liquid must increase to prevent the formation of a bulking sludge, i.e., poor settling qualities of the sludge that hinder retention of the bacteria in the secondary clarification process. For example, at high BOD/MLSS loading rates of F/M=1.0 or higher, it may be necessary to maintain 6 mg/L D.O. in the wastewater to prevent bulking. Pursuant to the gas transfer equation, this reduces the D.O. deficit driving force to 9−6=3 mg/L D.O., which more than doubles the unit energy consumption per unit of D.O. dissolved in the sewage solution.
Newer biological processes are able to considerably increase the concentration of biomass maintained in the activated sludge reactor. These processes operate at a high F/M loading rate of 0.5 to 1.0 lb BOD/lb MLSS-day. While the conventional aeration processes are only able to meet D.O. uptake rates of about 100 mg/L-hr, newer aerobic biological treatment processes with higher MLSS rates and higher F/M organic loading rates have D.O. uptake rates in excess of 200–400 mg/L-hr that can require D.O. concentrations above 4 mg/L. Therefore, in addition to the high energy costs and the high probability of D.O. loss through effervescence, there are other disadvantages and shortcomings of conventional aeration systems. These shortcomings include: (a) low achievable D.O. concentrations due to the rate of oxygen transfer when using air as the oxygen source and the corresponding energy required to dissolve oxygen in the absence of a large pressure gradient with the low (21%) oxygen composition in air; (b) long time periods when the mixed liquor sits in the microbial reactor tank; (c) floc shear; (d) high energy dissipation in the reactor; and (e) limited D.O. supply potential.
In an attempt to alleviate some of these shortcomings, high purity oxygen (HPO) has been used as an oxygen source. However, infusing high purity oxygen into wastewater is typically only able to maintain a D.O. of less than 10 mg/L in contact with the bacteria, as nitrogen and carbon dioxide stripping from the activated sludge dilutes the oxygen composition therein. Moreover, the most common high purity oxygen application requires that the biological reactor be fitted with an expensive gas-tight cover as opposed to the open-topped tanks used conventionally.
Overall, it is desirable to develop an oxygenation system for dissolving a gas into a fluid which: (a) has a low capital cost; (b) has a low unit energy consumption (kwhr/ton of gas dissolved); (c) discharges high D.O. concentrations; (d) has a high oxygen absorption efficiency; and (e) utilizes an open-topped tank for the reactor. Ideally, the system should be capable of producing D.O. concentrations in the activated sludge capable of sustaining optimal bacterial metabolism and have an oxygen absorption efficiency of at least 80%, all accomplished with reasonable capital costs, a low unit operating cost, and energy efficiency.